In recent years, there have been increasing demands for high-strength plated steel sheets used for automobiles or the like, and high-strength plated steel sheets with maximum tensile stress of 900 MPa or more is started to be used. As a method for forming vehicles or members of automobiles using such high-strength plated steel sheets, there may be mentioned bending such as press forming. Generally, the more the strength of a steel sheet is increased, the more the bendability worsens. Accordingly, when bending is performed on a high-strength plated steel sheet, there have been problems which occur easily, such as a crack inside a steel sheet at a deformation part, necking in an interface between a steel sheet surface and a plating layer, and destruction or peeling of a plating layer.
As a technique for improving bendability of steel sheet, Patent Document 1 proposes a high-tension hot-dip galvanized steel sheet in which a chemical composition of a steel sheet contains, in mass %, C: more than 0.02% and 0.20% or less, Si: 0.01 to 2.0%, Mn: 0.1 to 3.0%, P: 0.003 to 0.10%, S: 0.020% or less, Al: 0.001 to 1.0%, N: 0.0004 to 0.015%, and Ti: 0.03 to 0.2%, a balance includes Fe and impurities, a metal structure of the steel sheet contains 30 to 95% of ferrite by an area ratio, a second phase of the balance is constituted of one or more of martensite, bainite, perlite, cementite, and retained austenite, an area ratio of the martensite is 0 to 50% when the martensite is contained, and the steel sheet contains Ti-based carbonitride precipitates with a grain diameter of 2 to 30 nm and an average interparticle distance of 30 to 300 nm and contains a crystallized TiN with a grain diameter of 3 μm or more and an average interparticle distance of 50 to 500 μm.
Further, Patent Document 2 describes, as a hot-dip galvanized steel sheet excellent in bendability, a hot-dip galvanized steel sheet which has a chemical composition containing, in mass %, C: 0.03 to 0.11%, Si: 0.005 to 0.5%, Mn: 2.0 to 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0%, N: 0.01% or less and further containing one or both of Ti: 0.50% or less and Nb: 0.50% or less in the range satisfying Ti+Nb/2≧0.03 (Ti and Nb in this expression indicate the contents of respective elements (unit: mass %)) with a balance being constituted of Fe and impurities, has a steel structure having an average interval of 300 μm or less in a sheet width direction of an Mn concentrated portion extending in a rolling direction at the position of 1/20t depth (t: sheet thickness of the steel sheet) from a surface, an area ratio of ferrite of 60% or more, and an average grain diameter of ferrite of 1.0 to 6.0 μm and containing 100 or more precipitates per μm2 with a grain diameter of 1 to 10 nm in ferrite, and has tensile strength of 540 MPa or more.
Further, Patent Document 3 describes, as a hot-dip plated steel sheet having both ductility and bendability, a hot-dip plated steel sheet having a plating layer containing zinc on a surface of a cold-rolled steel sheet which has a chemical composition containing, in mass %, C: 0.08 to 0.25%, Si: 0.7% or less, Mn: 1.0 to 2.6%, Al: 1.5% or less, P: 0.03% or less, S: 0.02% or less and N: 0.01% or less and having a relation between Si and Al satisfying 1.0%≦Si+Al≦1.8% with a balance being constituted of Fe and impurities, and has mechanical characteristics satisfying TS≧590 (TS: tensile strength (MPa)), TS×El≧17500 (El: total elongation (%)), and ρ1.5×t(ρ: limit bend radius (mm), t: sheet thickness (mm)).
Patent Document 4 describes, as a cold-rolled steel sheet having good ductility and bendability, a cold-rolled steel sheet which has a chemical composition containing, in mass %, C: 0.08 to 0.20%, Si: 1.0% or less, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.005 to 0.5%, N: 0.01% or less and Ti: 0.02 to 0.2% with a balance being constituted of Fe and impurities, has a steel structure constituted of, in vol %, ferrite: 10% or more, bainite: 20 to 70%, retained austenite: 3 to 20% and martensite: 0 to 20% in which an average grain diameter of the ferrite is 10 μm or less, an average grain diameter of the bainite is 10 μm or less, an average grain diameter of the retained austenite is 3 μm or less, and an average grain diameter of the martensite is 3 μm or less, has mechanical characteristics such that a tensile strength (TS) is 780 MPa or more, a product (TS×El value) of tensile strength (TS) and total elongation (El) is 14000 MPa·% or more, and a minimum bend radius in a bending test is 1.5 t or less (t: sheet thickness), and has a sheet thickness of 2.0 mm or more, and describes that plating is provided on a surface of the cold-rolled steel sheet.
Patent Document 5 describes, as an alloyed hot-dip galvanized steel sheet excellent in bendability, an alloyed hot-dip galvanized steel sheet which has a chemical composition containing, in mass %, C: 0.03 to 0.12%, Si: 0.02 to 0.50%, Mn: 2.0 to 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0% and N: 0.01% or less and further containing one or both of Ti: 0.50% or less and Nb: 0.50% or less in the range satisfying Ti+Nb/2≧0.03 with a balance being constituted of Fe and impurities, and has a steel structure such that an area ratio of ferrite is 60% or more and an average grain diameter of ferrite is 1.0 to 6.0 μm, in which an alloyed hot-dip galvanized layer contains, in mass %, Fe: 8 to 15% and Al: 0.08 to 0.50% with a balance being constituted of Zn and impurities, and the alloyed hot-dip galvanized steel sheet has a tensile strength of 540 MPa or more and has excellent bendability.
Patent Document 6 describes as a high-strength hot-dip galvanized steel sheet excellent in workability, one having a hot-dip galvanized layer on a base steel sheet containing, in mass %, C: 0.03 to 0.17%, Si: 0.01 to 0.75%, Mn: 1.5 to 2.5%, P: 0.080% or less, S: 0.010% or less, sol. Al: 0.01 to 1.20%, Cr: 0.3 to 1.3% with a balance being constituted of Fe and inevitable impurities, and having a steel structure constituted of, in volume fraction, 30 to 70% ferrite, less than 3% retained austenite, and martensite of the balance, in which 20% or more of the martensite is tempered martensite.
Patent Document 7 describes, as an ultra-high-strength cold-rolled steel sheet excellent in bending workability, a steel containing, by wt %, C: 0.12 to 0.30%, Si: 1.2% or less, Mn: 1 to 3%, P: 0.020% or less, S: 0.010% or less, sol. Al: 0.01 to 0.06% with a balance being constituted of Fe and inevitable impurities, the steel having a soft layer of C: 0.1 wt % or less in a surface layer part on both surfaces by 3 to 15 vol % per surface with a balance being constituted of a complex structure of retained austenite of less than 10 vol % and a low-temperature transformation phase or further ferrite.